H5N1 avian influenza has long been a concerning virus. Since its discovery in 1996 in waterfowl, bird flu has occasionally caused isolated human cases that have quite often been fatal. But last year H5N1 did something strange: it started infecting cattle.
The absolute oddity of this leap may have been somewhat lost in the flood of bad news about H5N1, which by 2024 had already caused mass die-offs of seals and other marine mammals and which was simultaneously devastating chicken farms and causing periodic shortages of eggs. But infectious disease specialists were shocked. “Flu in cows is not really a thing,” says Jenna Guthmiller, a microbiologist and immunologist at the University of Colorado Anschutz Medical Campus. “If you would ask anybody that studies flu on their 2024 bingo card if they had, you know, mammary infection of dairy cows on there, no one would have.”
Influenza hadn’t previously been known to infect cattle, much less cause the kind of infections in their udders that have now begun circulating in milking parlors across the country. The continued circulation of H5N1 in cows is one of the biggest concerns experts have about this flu subtype. Though H5N1 hasn’t yet spread human-to-human, people can catch the disease from cattle, mostly through close contact with infected milk. And the more it circulates in an animal that humans regularly interact with, the more chances the flu has to stumble on just the right mutation to leap to people and start adapting into something with pandemic potential.